Structure of Selective and Nonselective Dicopper (II) Sites in CuMFI for Methane Oxidation to Methanol

The one-step valorization of natural gas remains a challenge. Methane conversion to methanol via chemical looping over copper-containing zeolites is a promising route, and CuMFI is among the earliest successfully applied. However, the structure of the active sites in CuMFI, as well as the effect of copper loading and Si/Al ratio on the copper speciation, are yet to be understood. We found that for CuMFI, the Cu/Al ratio determines the selectivity of methane conversion by governing the structure of the active dicopper sites. At a Cu/Al ratio below 0.3, copper-containing MFI materials host dimeric centers with a Cu-Cu separation of 2.9 angstrom and a UV/vis absorption band at 27 200 cm-1 capable of selective oxidation of methane to methanol in a wide temperature range (450-550 K). A higher Cu/Al ratio leads to the formation of mono mu-oxo dicopper sites with Cu-Cu = 3.2 angstrom, which exhibit a characteristic band at 21 900 cm-1 and react with methane at lower temperatures (<450 K), yielding overoxidation products. Identifying distinctions in the structure of selective and nonselective copper sites will aid in the design of better-performing materials.